RAINBOW ARCH, Utah, a superb natural arch of red sandstone spanning Bridge Canyon on the north slope of Navajo Mountain in the southeastern corner of the State, at a point four miles above where that canyon empties into the Colorado River 16 miles below the mouth of the San Juan River. The arch rises 309 feet from the bed of the stream below. The abutments are 278 feet apart and very large number of rays pass out, and an ob server at E sees a bright image of S in the direction EQ. This statement applies to any one color of sunlight; but, as the refrangibility increases from red to violet, the latter is bent more at P and Q, and the line EQ lies at a flatter angle. The observer, therefore, sees the violet rays reflected on drops at less altitude than those that reflect the red, the other colors being intermediate. The raindrop being spher ical, this reflection takes place in all directions, the fixed condition being the radius of the bow, that is the angle between the line from the observer to the bow and that passing from the sun to the observer, or, in other words, the ob server's shadow. For red light this angle is 42° 39', and for violet 40° 13'. If the sun were a luminous point each color would be sharply defined, but as the disc of the sun subtends an angle of about 30' each color is broadened to this amount, and they overlap.
Exactly similar reasoning explains the sec ondary bow (Fig. 2). The light that forms it
has been twice reflected, at R and at R'; the point Q lies above P, and rays entering either above or below P all emerge below Q. A glance at the diagram will show that the greater bend ing of the more refrangible rays makes the line its width on top is from 33 to 42 feet. It could easily span the dome of the Capitol at Washington and very nearly overtop the Flat iron building in New York. It has been made a national monument by the United States government. It is in a part of the Navajo country given over to the use of the Piute Indians, who called it Barohoini or rainbow. The Navajoes call it Nonnezoshe or stone arch. Near one abutment are remains of an Indian altar doubtless built for worship of the arch, for it is held in great reverence by the Indians. The arch is eroded from buff colored fine-grained sandstone, brick red upon its sur face but stained darker in vertical streaks, a part of the upper La Plata sandstone a Jurassic Age. It was discovered by W. B. Douglass of the United States Land Office on 14 Aug. 1909. The arch may be reached from Gate, Utah, 37 miles distant in a straight line, by several days of hard riding. Gate is 180 miles north of Gallup on the Santa Fi Railroad or 140 miles from Dolores, Colo. Consult Pogue, J. E (in National Geographic Magazine, Vol. XXII, pp. 1048-56, 1911).